The present invention relates to a paper which is suitable as fiber reinforcement in a base layer of a packaging film, in particular a tubular, cellulose-based artificial sausage casing. The present invention also relates to a packaging film, in particular to a tubular artificial sausage casing, with a base layer comprising fiber-reinforced cellulose.
Films with a base layer based on fiber-reinforced cellulose are produced conventionally by the viscose process. In this process, a paper, for example of hemp fibers, bent into the form of a tube is, in general, coated and impregnated on one or both surfaces with an alkaline viscose solution which, as is known, contains cellulose xanthate. The paper treated with viscose is then treated with an acidic coagulation fluid, whereby the cellulose xanthate which is soluble in the alkaline region is precipitated. In a further treatment with acidic regenerating fluid, the regenerated cellulose hydrate is formed. In the end product, the paper is completely covered on one or both sides with a cellulose layer and forms the so-called fiber reinforcement of the seamless cellulose tube. It is also known for the production of flat films, to treat the paper in the form of a web, that is to say in the flat state, with viscose and to carry out the coagulation and regeneration on the viscose-treated paper web in the above-described manner. The web-shaped cellulose film, reinforced by the paper, is then bent into the form of a tube, if desired, and the overlapping edges are adhesively bonded, sealed or sewn to one another, a longitudinal axial connecting seam being formed. Tubular fiber-reinforced cellulose casings are very widely used as artificial sausage casings.
The paper which serves as the fiber reinforcement in the packaging film is produced in the conventional manner from cellulose fibers. From U.S. Pat. No. 3,135,613 it is known, in the production of the paper, to coat the cellulose fibers, laid down in the form of a web, with a dilute alkaline viscose solution and then to dry them, the cellulose being partially regenerated. The cellulose present as cellulose xanthate in the viscose solution can also be regenerated by the action of an acidic fluid. The coating of regenerated cellulose, formed on the cellulose fibers, is so thin that the porous structure of the paper is preserved. The cellulose coating serves as a binder and is intended to increase the wet strength of the paper.
In the production of fiber-reinforced cellulose film by the viscose process, however, this paper has the serious disadvantage that the cellulose coating serving as binder for the fibers is not sufficiently resistant to alkaline substances and to hydrolysis. During the production of the fiber-reinforced cellulose film, a further coating and impregnation of the paper with alkaline viscose solution takes place. Under the action of this viscose solution on the paper the cellulose already present as binder for the paper fibers is partially dissolved, so that the paper fibers are no longer sufficiently firmly joined to one another. The resulting film then does not show the required strength; rather, tubular films having a fiber reinforcement of this paper tend to burst even under a relatively low internal pressure.
British Patent No. 1,091,105 discloses a process for producing fiber-reinforced cellulose films, wherein the paper serving as reinforcement contains a curable synthetic resin, for example a polyamide cross-linked with epichlorohydrin. The paper prepared with the resin is preferably composed of hemp fibers, and it is coated and impregnated as usual with alkaline viscose solution. U.S. Pat. No. 3,484,256 also teaches a treatment of paper with such resins before the coating with the viscose, and a polyacrylamide is also to be used additionally in order to increase the strength of the paper. It has been found in practice, however, that even cellulose casings, which contain these papers as reinforcing material, cannot ensure the particularly high bursting strength demanded for certain types of sausage.